Fraser Horton
Fraser Horton
Last Updated on December 15, 2020

The most important factors for cannabis cultivation are air conditioning, grow lights, and maintaining the right relative humidity. While growers generally know how to control room temperatures, they also need to focus on maintaining the right relative humidity, good air circulation and ventilation, and proper spacing between the plants

By growing your marijuana plant in the right conditions, you can avoid the most common problems that come from low and high humidity. Among them are dried out plants, cell death within the plants by closing the pores in order to retain moisture, developing root rot, bud rot, and other unwanted conditions, diseases, and bacteria.

If you want to start cultivating cannabis, humidity is one of the most important topics for growers who cultivate indoor marijuana. But why is humidity so important for optimal plant growth when growing your cannabis plants? We’ll answer that question as well as go into what conditions you need to make your grow room a cannabis thriving heaven.

What Are Relative Humidity Levels When It Comes to Marijuana Plants?

Relative humidity (RH) is the amount of water vapor in the air which can vary depending on temperature. In general, hot air holds more moisture compared to cold air, and for that reason, it’s important to measure the humidity level in the growing environment at every growing stage. The fluctuation of the relative humidity levels results in plants experiencing unwanted side effects in the growing stages. 

Examples of this include closing the stomata and slowing down evaporation in order to prevent the loss of moisture which can result in plants having stunted growth. Moreover, low humidity environments cause plants to consume a lot of water through the roots and uptake high levels of nutrients, which can result in nutrient burn and yellow burnt tips. In high humidity environments, plants can experience powdery mildew which is a fungal disease where white powdery spots appear on the leaves and stems of plants.

The ideal relative humidity levels vary from plant to plant. For marijuana plant cultivation, it’s important to keep in mind the stage of development the plant is in and tailor the RH from there.

Indica vs Sativa Humidity Levels

Different cannabis strains require different cultivation temperatures and humidity levels. Indicas originate from dry, mountain regions, and thrive in low temperatures. Sativas on the other hand, naturally grow in warm, subtropical climates and prefer high temperatures and more humid conditions.

Nowadays, cannabis is a crossbred plant, and a lot of strains are Indica-Sativa hybrids. As a result, growers assess their plants and experiment with RH levels in order to get to the right one for their hybrid plant.

Vapor Pressure Deficit

Vapor pressure deficit (VPD) is also an important term for marijuana growers. VPD measures the temperature and relative humidity so cultivators can calibrate it to enable optimum plant growth. The ideal VPD for greenhouse growing for most plants according to Wikipedia ranges between 0.8 to 0.95 kPa, with the ideal being around 0.85 kPa

For plant-growers, the temperature as well as the RH are important in order to get the best results when growing marijuana.

  • Air that is too hot and dry results in a high VPD, so plants will have a stretched growth.
  • Air that is too cool and humid results in a low VPD, so plants might have problems with fungus or mold.

Optimal Grow Room Humidity Levels for Indoor Cannabis Growing

The life cycle of marijuana plants contains various growth stages, and each has special conditions in order to get the cannabis seeds from the seedling stage to the flowering stage. Here we’ll list the ideal humidity levels for each growing phase for your indoor cannabis cultivation. To measure this it will be a good option to use a quality thermometer and hygrometer to measure the temperature and humidity in your grow room or use the thermo hygrometer which measures both.

Germination Stage

This stage requires a relative humidity of 80-90% and a temperature of 25-26ºC during the day and 21-22ºC at night, and it applies for seeds that have just sprouted and young seedlings from 0 to 10 days old. Plants are pretty fragile at this stage and need high moisture levels in order to develop strong roots in the next stages. 

Cloning Stage

Cloned marijuana plants require different conditions than regular marijuana seeds since clones don’t have roots. Until they grow roots they obtain moisture through their leaves, and for that reason, they should be cultivated in high humidity 80% to 100%. Once the plant forms roots, the humidity can be lowered to around 60%.

Seedling Stage

For seedlings, it’s crucial to maintain a correct relative humidity especially until they develop roots between 12 and 30 days. Similar to clones, they thrive in higher humidity with levels of 65-80%. Since they have weak root systems, increasing the humidity levels will allow them to take up more water and develop strong roots. At this stage, the room temperature should be around 77°F (21°C ) during the day and 70°F (21°C) at night.

Vegetative Stage

During the first 3 to 4 weeks of flowering, vegetative plants prefer moderate humidity levels between 55–70% depending on the strain with daily temperatures between 71–82°F (22–28°C), and temperatures around 64–75°F (18–24°C) at night. As plants develop stronger roots during this stage, growers tend to drop humidity levels. At the end of the veg phase, the levels need to be around 50-60%.

Flowering Stage

In the early flowering stage plants tend to benefit from lower humidity levels which should be around 40–50%, and occasionally increase the humidity levels to 55%. The ideal temperature for this stage is 68–78°F (20–26°C) since high temperatures can slow down bud growth and maybe reduce the potency of the buds by causing terpenes and cannabinoids (like THC and CBD) to burn off.

Late Flowering Stage

In the two to three final weeks, the optimal humidity should be around 30–40% and the temperatures should vary between 64–75°F (18–24°C) during the day and 60-69°F (16-20°C) at night.

Outdoor Cannabis Grow

When growing cannabis outdoors the humidity isn’t an issue. Spring and summer offer higher humidity, and moisture from morning dew will evaporate during the day. When summer ends, the flowering stage starts, and cooler days follow with more rainfall and humidity. A bit of rain might not cause the buds to rot at this stage, but once you get to the blooming stage, the plants will benefit from brushing the dew off in the mornings.

Growers Tips and Tricks for Growing Cannabis Plants

When growing cannabis, you might experience some of these typical growing cannabis problems due to high or low humidity so you can control humidity in your grow space. 

To Lower Humidity

  • Use a dehumidifier.
  • Use an air conditioner to remove the moisture from the air and increase the cool air supply.
  • Run the exhaust vent on high in order to push hot air from the room.
  • Water the plants at the beginning of the light period.

To Increase Humidity

  • Use a humidifier.
  • Increase the room temperature in the grow space.
  • Turn down the exhaust vent to keep more warm air.
  • Mist the plants but not the flowering plants in order to avoid bud rot.

Final Thoughts on Growing Marijuana and Ideal Humidity

The ideal humidity for growing marijuana plants depends on the growth stage the plant is in, so growers will need to modify the relative humidity as they modify the temperature. Great results can be achieved with high humidity levels, but you need to know whether your plant is a Sativa or an Indica. Sativas thrive in higher humidity, while Indicas can develop bud rot when exposed to those conditions. As a grower, it would be a good idea to invest in a hygrometer to monitor the humidity in the grow room.

Disclaimer

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